If you have messaging included in your plan there is no additional charge, If you don't you will be charged the normal text messaging rate of $.20

Sending a Text-to-Landline message is just like sending a standard text message. Just create a text message from your Sprint Nextel phone, and address it to a 10–digit phone number as you would with a regular text message. Your text is converted to a voice message, which is automatically read to the recipient when he or she answers the phone. If the recipient does not answer the phone, your text message is stored as a voicemail, provided the recipients have voicemail service on their landline phone. Your recipient can also reply to your message.